This invention relates generally to heat exchangers and more particularly to heat exchangers of the plate-type, ie., wherein thermal energy is transferred between two currents of moving fluid through a plate formed of heat conductive material.
Although in the following description reference will be made to air-to-air heat exchangers wherein thermal energy is transferred from a warm air current to a cooler one, it is understood that the invention is applicable to heat transfer between a pair of any fluids. The present invention has its greatest applicability in the area of air-to-air heat exchangers which are intended to recover energy from a waste or stale air steam and transfer it to an incoming stream of fresh air.
There are presently available many types of air-to-air heat exchangers which function to recover thermal energy from stale or waste air and transfer it to incoming fresh or make up air. For example, heat exchangers of the rotating wheel regenerative type, the heat pipe type, the run around coil loop type, the shell-and-tube and the plate-type are all known and each such type of heat exchanger has certain peculiar characteristics which tend to make it more readily adaptable to a particular application than the other devices. Of all these types of heat exchangers, the plate-type heat exchanger is acknowledged as being the most simple in construction as well as being one of the more efficient and easier to maintain types of heat exchangers. The present invention relates to such plate-type heat exchangers.
Plate type heat exchangers can generally be divided into two categories. More particularly a first category can be termed "discrete plate exchangers". Such discrete plate heat exchangers generally are formed of a plurality of adjacent, individual sheets or plates formed of a heat conductive material and which extend generally parallel to each other between the open ends of a housing so that a plurality of adjacent channels are defined between pairs of adjacent plates
The second category of plate-type heat exchangers can be termed "continuous sheet exchangers" wherein the thermal transfer core is generally formed of a continuous sheet of heat conductive material which is folded upon itself in opposite directions alternately to define a plurality of spaced, parallel sheet portions. In this manner a plurality of channels are formed between adjacent sheet portions.
Continuous sheet heat exchanger have several distinct advantages relative to discrete plate exchangers. More particularly, by forming the thermal transfer core of a single sheet of conductive material, significant economies in manufacturing are achieved relative to discrete plate exchangers which as mentioned earlier require the manufacture of a plurality of separate plates. The continuous nature of the thermal transfer core of a single continuous sheet heat exchanger reduces the extent of sealing required in the manufacture of the heat exchanger. Further, it is possible in continuous sheet exchangers to achieve the proper spacing between adjacent pairs of sheet portions through the provision of appropriately formed spacing dimples in the continuous sheet. A particularly favorable arrangement of such spaced dimples is illustrated in U.S. Pat. No. 4,043,388.
However, conventional plate-type heat exchangers of both the discrete and continuous type have certain disadvantages. Of perhaps the greatest significance, currently available discrete and continuous counter flow plate-type heat exchangers require significant amounts of energy to force the two currents of fluids through the heat exchangers. The resistance to flow is due to restricted sized openings in the heat exchangers. The restricted sized openings is a result of heat exchanger designs. One current of fluid enters one end of a heat exchanger and exhausts at an opposing end. A second current of fluid enters the opposing end, flows in counterflow direction and exits at the end the first fluid enters. A particularly efficient design of a counterflow heat exchanger is disclosed in U.S. Pat. No. 4,314,607, the disclosure of which is hereby incorporated by reference. In the typical counterflow heat exchanger, two sides of the heat exchanger are blocked so that fluids cannot enter and the end areas are each divided into halves, one half for entering fluid and one half for exiting fluid. Other heat exchanger designs include the UI, LU, ZI, UU, LL, UI, LU, ZI, UU, LL and X flow. The letters designate the flow pattern through the plate type heat exchanger. Only the X flow utilizes four open sides of the heat exchanger for free air flow, but being a cross flow heat exchanger, is less efficient than a counterflow heat exchanger. Plate type heat exchangers having the various flow patterns are disclosed in the Thermo Z.RTM. High Temperature Heat Exchangers brochure available from Des Champs Laboratories Incorporated (DLI), Box 220, Douglas Way, Natural Bridge Station, VA 24579.